There's only one reason celebs go on shows like Letterman or Leno: to get publicity for their latest movie, product, endorsement, etc. It's a "you use me, I'll use you" scenario.
Paris was using Letterman's show to pitch her latest perfume and movie. She obviously wants to portray herself as a savvy business woman and a serious actress. Dave was merely extracting his side of the deal with his interview. If she was a savvy as she wants to portray, she should have seen that interview coming from a mile away. He was right to call "bullsh*t".